This application claims the priority of German patent application no. 100 13 435.1, filed Mar. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a lamina stack for a two-pass evaporator composed of laminae for converting two media mass streams from a liquid state into a gaseous state.
In principle, evaporators composed of laminae are known. One such evaporator is described for example in German patent document DE 44 26 692 C1. The two-stage (but not two-flow) evaporator unit described there serves to convert a liquid reactant mass stream, which can be adjusted as a function of a set, into a gaseous reactant mass stream. The liquid reactant mass stream evaporates at least partially in a first stage with the aid of a heat transfer medium and, if required, completely evaporates in a second stage and is then superheated. In so doing it is proposed that the evaporator unit be constructed by alternate stacking of laminae comprising heat transfer medium channels and of laminae comprising reaction channels, with at least a first and a second stage integrated in each lamina. The first stage is formed as a channel with a minimized cross-sectional area, which directly adjoins the inflow line, and is operated at high heat transfer values and. The overall cross section of the reaction channels in the second stage increases in the flow direction.
Also known in the general prior art are so-called two-pass evaporators. This means that the evaporator, which is heated by a heat-transfer medium or is subjected to direct heating, has two separate chambers for evaporating two separate reactant mass streams or media mass streams. In this case, each of the regions for the respective medium has at least one dedicated inlet region, but if the two media streams already mix in the outlet region of the evaporator and flow out through a common outlet region, one would nevertheless speak of two-flow evaporators.
If implemented in a plate or laminae design, these two-flow evaporators have a serious disadvantage owing to their construction. That is, the two media chambers of the two-pass evaporator are normally combined into one lamina stack, which is supplied with thermal energy by a heat-transfer medium, a catalytic combustion or the like. Recessed in the plates or laminae are passages, in which the media streams flow. Conventionally, large regions of the channel sections are arranged one above another, so that the supply of thermal energy through the lamina can take place virtually from one side only, as the already vaporous medium in the channels of the bottom lamina insulates the upper lamina at least partially with respect to the energy coming from below. Due to this fact, there is a major risk of formation of cold regions in which the medium, especially in situations where instead of closed ducts predominantly open structures are used on the laminae, forms regions in which the temperature is so low that the medium does not evaporate, or evaporates only to a limited extent. Thus it is possible for cold through-flow regions to be established, so that medium with a high fraction of liquid particles may exit from the evaporator.
It is therefore an object of the invention to provide a lamina stack for a two-pass evaporator composed of laminae which, using a simple and compact design, enables a distribution of the supplied thermal which is as even as possible.
Another object of the invention is to provide such an evaporator which has a small overall size, and is capable of uniform transfer of a high evaporator load.
These and other objects and advantages are achieved by the lamina stack according to the invention, which includes at least two laminae. Structures comprising recesses and raised regions on a first media lamina form a first media chamber perfused by one of the media mass streams. The structures on a second media lamina form a second media chamber perfused by the other one of the media mass streams. The at least two laminae can be supplied with thermal energy.
Recesses of one media lamina are disposed above or below raised regions of an adjacent media lamina. This structure ensures that over a major part of the lamina, the recesses will be surrounded by as many raised regions and areas of the laminae as possible. As these raised regions and areas of the lamina provide very good conduction of the thermal energy introduced into the lamina stack from outside, the recesses into which the media mass streams are flowing are supplied very efficiently and very uniformly with the thermal energy.
In a further, highly advantageous refinement of the invention, projection areas of the raised regions of the two media laminae overlap. A plan view of the laminae (a view perpendicular to the plane in which the laminae are situated) shows the projection areas of the raised regions of the respective lamina. It has now been found, surprisingly, by the inventors that the efficiency of the evaporation increases greatly if the raised regions of the two media laminae are offset relative to one another and if the projection areas of these raised regions at least touch, but ideally overlap in each case over a wider range.
The lamina stack for the two-pass evaporator according to the invention is particularly suitable if it relates to an evaporator heated by means of a gaseous heat transfer medium. The risk of overheating of these regions, which cannot be cooled by the media stream, is then smaller than with a comparable evaporator heated by means of a liquid heat transfer medium.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.